From: Joel VanderWerf <vjoel@...>
Date: 2006-02-08T08:58:50+09:00
Subject: Re: marshalling constants

Brian Buckley wrote:
>>
>> class A
>>   A1 = new
>>
>>   def self.name_for_constant obj
>>     @name_for_constant ||= {}
>>     @name_for_constant[obj] ||= constants.find {|c| const_get(c) == obj}
>>   end
>>
>>   def _dump(limit)
>>     self.class.name_for_constant(self)
>>   end
>>
>>   def self._load(str)
>>     const_get str
>>   end
>> end
> 
> 
> 
> Whoa.  I see it works (for the A constants only as written, getting
> TypeErrors on A non-constants).  I don't understand why it works just yet.
> Will examine further tonight.
> 

I see your point. This implementation can't marshal anonymous instances:

anonymous = A.new
Marshal.dump(anonymous) # ==> type error

One way around this is to subclass (instances of the subclass are the
only ones that can be assigned to constants), but that may not be
acceptable to you.

class A
  #...
end

class AConstant
  A1 = new

  def self.name_for_constant obj
    @name_for_constant ||= {}
    @name_for_constant[obj] ||= constants.find {|c| const_get(c) == obj}
  end

  def _dump(limit)
    self.class.name_for_constant(self)
  end

  def self._load(str)
    const_get str
  end
end


before = AConstant::A1
after = Marshal.load(Marshal.dump(before))
puts before == after
puts before.object_id == after.object_id

anonymous = A.new
p Marshal.load(Marshal.dump(anonymous))


It would be better to put some logic in the _dump and _load methods to
handle the two cases in one class. The problem is that there is no way
to call "super" in _dump to handle the case of anonymous objects, which
should be dumped in the "normal" way. This is unfortunately the way
marshalling is structured in ruby...

-- 
      vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407